Rare earths are widely used in various fields such as industry, military and agriculture, the rare earth functional materials developing rapidly in the recent decades have played an irreplaceable role in the high and new technical industries as well as military and astronautic technologies, which makes rare earths become a globally recognized strategic resource. Rare earth is a non-renewable natural resource, but the demand for rare earth is ever-increasing due to the strengthened effort for developing its materials. However, the high-grade rare earth ores and rare earth reserves are dwindling. Therefore, it has attracted extensive attention and researches on the technologies of effectively extracting rare earth with respect to low-grade rare earth mines and the technologies of recycling rare earth from wastewater containing a low concentration of rare earth which is generated during the production process. Given that the requirements of protecting resources and ecological environment become rigorous, it is obviously urgent to research and develop the recycling technology with respect to solution containing a low concentration of rare earth and the technology of treating wastewater to reach standard of effluent discharge.
The mining wastewater produced accompanied exploitation process of the ion adsorption rare earth deposits and subsequent trickle leaching process with rain water generally contains a low concentration of residual rare earth and a certain concentration of ammonia nitrogen, and the large amount of waste water are difficult to be treated. For example, the analysis result of the river water in a rare earth mining area in Jiangxi Province in May illustrates following content: the concentration of rare earth calculated by rare earth oxides (REO) is 30 mg/L, the concentration of ammonia calculated by nitrogen (N) is 51 mg/L. If the residual rare earth has not been recycled, which is bound to accelerate the loss process of rare earth, and result in an increased content of rare earth and electrolyte in the water environment in the mining area and even the downstream living quarters, thus the ecological environment is destroyed, and the water safety is under threat. Because of the widespread presence of weathering crusts, there are many ion adsorption rare earth deposits on a global scale which are similar with those reserved in the southern China, while its content of rare earth is as lower as 10-300 mg/kg. Although the weathering crusts with minimal content of rare earth have not been exploited as the rare earth resources, it will become a kind of potential rare earth resources once the amount of other rare earth resources is reduced and the mining cost goes up. Moreover, the use of electrolyte in the producing processes of other industries and agro forestry will lead to the leaching of said ionic rare earth, the leached rare earth has the value of recycling use; if the leached rare earth is not treated, it will lead to environmental pollution either. In addition, given that the rare earth ions possess non-biodegradable property, the rare earth ions which are latent in the environment for a long time may enter human body by passing up the food chain, and the chronic accumulation of said rare earth ions in human body will result in various diseases and physiological disorder, thereby endanger human health.
Furthermore, the wastewater containing a low concentration of rare earth ions are produced in the rare earth separation enterprises and in some mineral processing and smelting processes, thus it is necessary to treat said wastewater and recycle the rare earth contained therein. For example, the wastewater contains enriched rare earth elements and a certain concentration of accompanied ammonia nitrogen in the procedure of processing minerals, for instance, bauxite, phosphate ore, zircon sand and ilmenite. The weathered clays associated with fluoride carbonate rare earth mines also contain a certain amount of rare earth, and some rare earth dissolves into solutions in the process of mineral separation and the contact process between the rare earth ore with the aqueous solution containing acid and ammonium. An indiscriminate discharge of these solutions containing rare earth will not only result in a waste of rare earth resources, but also bring forth severe threat on the ecological environment.
The methods of enriching and recycling rare earth have been widely researched and applied. At present, the methods primarily include precipitation, extraction, reverse osmosis and ion exchange resin, and the precipitation method is the simplest process among them. The precipitation process generally relates to neutralize the wastewater containing rare earth with lime such that the solution manifests alkalinity, thereby the rare earth is precipitated out in the form of hydroxide and is separated from a large amount of water. The pH value of large amount of water shall be regulated by adding into acidic substance so as to obtain the neutral solution available for discharging, and the formed sediment is in a colloidalstate with tiny particles, and thus it is difficult to recycle the rare earth; moreover the presence of calcium ions will impose pressure on the subsequent process of purification. Resin adsorption method is relatively simple, but it has the disadvantages such as the loading amount of resin is small, cost of resin is high, and the rare earth is hardly desorbed. The extraction method is relatively complicated, while the method has virtues of high efficiency and enrichment factor, it gives rise to many problems, for instance, its phase ratio is too small, the extraction agent suffers from severe loss in the course of dissolution, the cost is high and secondary pollution is serious.
At present, the methods for treating ammonia nitrogen in wastewater are mainly as follows: air stripping method, biological method, chemical precipitation method and break point chlorination method. The air stripping method can be used for efficiently removing nitrogen, but the ammonia nitrogen is not fundamentally removed. The biological method can produce a desirable effect, but the processing time is relatively long and the treatment process in the practical application is difficult to be controlled. Chemical precipitation method can be operated in a flexible manner and generate a desirable effect of removing ammonia nitrogen, while its cost is relatively high. The break point chlorination method is a very effective method, its reaction process may be illustrated as follows: NH4++1.5HOCl→0.5N2+1.5H2O+2.5H++1.5° Cl−. The break point chlorination has the advantages that its treatment effect is stable and the process is not affected by temperature, and the equipment investment is low, and reaction is rapid and complete, and the process has an effect of disinfection. However, it imposes a high requirement on controlling the pH value, thus pH value control is the difficult point of applying the break point chlorination method.